They may be polymer but they aren't fragile though. They will last.
Are you used to using aperture rear sights? It takes getting used to but you grow to like it. You can use the smaller aperture for distance and accuracy, and the larger aperture for close range or dark light situations. Odds are you'll be using the RDS normally and have the sights folded down most of the time so I'd give the MBUS a little more time. If you do decide you want something else in the rear you could continue to use the front MBUS. Of course in the end it's up to you. I'll just say I'm very impressed with the rear MBUS Gen 2 I have. (Front is fixed A2.)
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek